Audio-frequency compensator circuit



July 3, 1945. J. L.. HATHAWAY AUDIO FREQUENCY COMPENSATOR CIRCUIT FiledNOV. 12, 1943 WMV RW Y UM/OM T7 N4 R WH m4 .A mm 7 M J n Ww l 7.

Patented July 3, 1 945 UNITED STATES PATNT orrics AUDIO-FREQUENCYCOMPENSATOR CIRCUIT Jarrett L. Hathaway, Cambridge, Mass., assignor toRadio Corporation of America, a corporation of Delaware ApplicationNovember 12, 1943, Serial No. 510,008

13 Claims.

(Cl. 17h-100.4)

dicated diagrammatically several circuit organizations whereby myinvention may be carried into effect.

In the drawing:

Fig. 1 schematically shows a circuit diagram embodying the invention,

Fig. 2 graphically shows the functioning of the tone compensationnetwork,

Fig. 3 shows a modication of the compensation circuit, adapted to beapplied to a record play-back system wherein the pickup moves from ery.Such changes in frequency response also occur during the sound recordingprocess, and the response characteristic, as a function of time, dependsupon whether the cutter' travels from the periphery toward the center ofthelrecord, or in the reverse direction. q

Accordingly, it may be stated `that it is one of the main objects of mypresent invention to provide a novel method of, and means for,automatically compensating for the frequency response characteristic ofa record disc as a function of diameter, and the compensation beingadapted for either the recording or play-back process to make up for therelative loss of high audio frequencies where the record velocity islow.

A more specific object of my invention is to provide a tone compensationnetwork in the output circuit ofthe electric pickup of a phonographrecord play-back system, such tone compensation network comprisingessentially an elecu tronic impedance whose magnitude has apredetermined relationto the record speed.

Yet another object of my invention isto provide in an audio frequencytransmission line an automatic tone compensation network consisting ofan electron discharge device whose input impedance is related in apredetermined manner to the rate of disappearance of a negativeblasdeveloped in the input circuit ofl the discharge device. l

Still other objects of my invention are to improve generally theefficiency and reliability of record reproducing circuits,v and moreespecially to provide automatic tone compensation networks for soundrecord systems which are not only reliable in operation, but areeconomically manufactured and assembled.

I'he novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims; Ythe inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have inthe center of therecord toward'the periphery.

Referring, now, to the accompanying drawing, wherein like referencecharacters in the dierent figures designate similar circuit elements,there is shown in Fig. 1 in a purely schematic manner aturntable iadapted to be driven by a conventional motor 2 energized from currentsource 3' through conductors 3. The numeral t denotes the spindle of theturntable'upon which is properly centered the record disc 5, which maybe either a master `disc upon which a sound recording is to be cut or arecord which is to be played back. The numeral t denotes the stylus inthe case of a play-back system. or it may be the cutter in the case ofsound recording.

The numeral l designates the usual inductance coil which is associatedwith the stylus,` there being generated across coil l the audiofrequency currents corresponding to the modulation of the sound groovesof record 5. In the latter case, of course, we are dealing with aplay-back system. The invention is not limited to the particular pickupshown. For'the sake of simplicity let it be assumed that record 5 hasgrooves corresponding to a desired audio frequency selection, while S-Jlis the usual electric pickup which is employed to convert the mechanicalvibrations of the stylus 6 into corresponding audio frequency currents.As is well known to those skilled in the art, the electric pickup ismounted at the free end of a tone arm.

The record may be played back in either of two Ways. The stylus pointmay be positioned at the periphery of the record and travel toward thecenter-of the record in a radial fashion, as is generally the case. Onthe 'other hand the play# back path may be radially from the center ofthe record toward the periphery.` In either case the audio frequencycurrent developed across coil l is transmitted over an audio frequency'transmission line 8 to the input electrodes of an amplier tube. Thelatter may be followed by one or more separate audio frequency amplifierstages terminated by a reproducer. These are networks so well known tothose skilled in the art that it vis not necessary to illustrate them inthe drawing. In Fig. 1 the low `potential sides of the input and outputterminals of the transmission line are shown at ground potential.

As is well known, there occurs a relative loss of high audio frequencieswhen the stylus 6 rides toward the center of record 5. This occursbecause the relativerecord velocity at that point is low. The linearvelocity of any rotating disc is greater at its p eriphery than close toits center. Hence, to the ear of the listener there occurs an apparentdiminution of the higher audio frequencies. According to my invention,an automatic compensation is secured for this apparentloss of high audiofrequency response. Compensation circuits elsewhere in the system,whether play-back or recording, provide sufflcient high frequencyaccentuation to attain the desired response characteristic over theentire record.

The automatic tone compensation action is' secured by providing across'the transmission line an electronic impedance which normally nected toground through an unbypassed resistor 1l. The control grid I2 isconnected by the direct current blocking condenser I3 to the junction ofa pair of series-connected isolation resistors Il and I5. Condenser I3is suillciently `large to offer substantially zero impedance at thelowest reproducible frequency. The isolation resistors are arranged4 inthe high potential side of the audio transmission line. These isolationresistors may be dispensed with, if desired. 'Ihe plate I6 of tube 9isiconnected to a point of positive potential of a direct current source(not shown). 'I'he screen grid of the tube is connected through avoltage reducing resistor I1 to the aforesaid positive potential point.'I'he condenser I8 bypasses the screen grid to ground.

Control grid I2 is connected to a point of predetermined negativepotential relative tol ground through a path which comprises agrid-return resistor I 9 and a switch 2I. The negative bias source isschematically represented. Any suitable source of negative bias may beused. Thev lower end of resistor I9 is connected tothe grounded end ofcathode resistor I I through a resistor 22 which is shunted by condenser23. Resistor 22 may be made adjustable for -a reason trol grid I2 isalso directly connected to cathode I by condenser 30.

In my U. S.' Patent No. 2,323,598, granted July 1 6, 1943, I havedisclosed and claimed an electronic impedance device of the type showninFig. l. In that patent there has been described a network having anelectron discharge vdevice provided with at least -a cathode, controlgrid and plate, the cathode including in circuit therewith an'unbypassedresistor. An external impedance element is connectedbetween the con--ses . which willbe hereinafter set forth. The con-y trol grid andcathode. It has been .shown in that patent that the eiective impedancebetween the control grid of the tube and the low potential side of thecathode resistor is a function of the gain of the tube.-

More specifically, it has been explained in my said patent that if theproduct of the impedance pedance between control grid I2 and ground ismultiplied by the magnitude of the tube 'transconductance. Preferably,the impedance of-'condenser 30 should be'greater than the resistance ofresistor II. In brief, the transconductance of tube 9 is used to reducethe eective impedance offered by an element connected across grid andcathode. Condenser 30 is the element whose impedance is controlled.

When the switch 2| is momentarily closed, the condenser 23 of the' timeconstant circuit Rza-C23 is charged up to a predetermined value.Preferably, the bias applied from the negative bias source has amagnitude such that when switch 2| is closed control grid I2 will beinstantly biased to the plate current cut-olf point. In other words, theeffective impedance between control grid I2- and ground will be aminimum if the transconductance of tube 9 is a minimum.

However, as the transconductance increases the effective impedancebetween control grid I2 and ground also increases. Since the eiectiveimpedance of control grid to ground of tube 9 is connected across theaudio transmission line it will be seen that the attenuationvof thehigher audio frequency components depends upon the transconductance oftube 9. The constantsof Raz-C23 are chosen that the initial approxi'-mately cut-olf bias applied to grid I2 gradually disappears as thestylus 6 rides in the sound grooves toward the center of the record. Inother words, the higher audio frequencies are attenuated to a maximumextent when the stylus pointjs atv the vperiphery of the record,

while the attenuation is a minimum when the stylus has traveled towardthe center of the record. Another way of explaining theV action istostate that there is provided a form of equalizingaction by initiallyrobbing the pickupl The motor switch 4o and the bias switch 2| may beconjointly actuated as suggested by the dotted line 4I, although in manycases the motor should be separately controlled. In otherwords, switch2| may be adjusted to be momentarily closed when the motor 2 isenergized thereby to start the record play-back. Immediately a negativecharge isv given to Vcondenser 23. `Switch` so chosen that complete, ornearly complete, discharge will have occurred during the length of timerequired for playing the record back.

Where the record play-back is from the center of the record toward theperiphery, then the biasing arrangement shown in Fig. 3 isemployed inthe grid-cathode circuit of tube 9. In this case, the switch 2ifunctions to short out 'the negative bias from a suitable direct currentsource 5d. In other words, the attenuation of the higher audiofrequencies is a minimum at t e beginning of record play-back, whereasthe attenuation increases toward the-periphery of the disc by virtue ofthe fact that the bias from Vnegative voltage source 50 is built up asthe stylus approaches the periphery of the disc. It isto be understoodthat the functions and operation described heretofore are equallyapplicable to the master disc. In that case, the audio frequencycurrents will be derived from theright-handside of the transmission linein Fig. l1. However, the circuit arrangement will otherwise be the same.It is to be understood, of course, that the R-C value of networki23-22may readily be adjusted for different types of record diameters by ad`justing the relative magnitudes of resistor 22 and condenser 23 in apredetermined manner, the time constant of the'circuit becoming longeras the product of the resistance -and capacity is increased. Y 4

-Whi1e I have indicated and described several systems for carrying myinvention into effect; it will be apparent to one skilled in the artthat my invention is by -no means limited to the particularorganizations shown and described, but that many modifications may bemade without V departing from the scope of my invention, as set forthinthe appended claims.

What I claim is: Y 1. In combination with a source oi' audio frey 3 thevoltage charging of said time constant network -by said negative voltagesource.

3. In combination with an audio frequency `transmission line havinginput and output terminals, an attenuation network comprising anelectron discharge tube having at least an anode, a cathode and acontrol grid, an unbypassed resistive impedance arranged in the spacecurrent path of said tube between the cathode and ground, a condenserconnected between the control grid and cathode, said condenser andresistive impedance being in series'relation, a resistor and secondcondenser connected in shunt relation between thecontrol grid'and theground-` ed end of said resistive impedance, means adapted .to apply ,anegative voltage of a predetermined magnitude across said secondcondenser, said shunt condenser and resistor providing a time constantnetwork adapted to dissipate said charge after` the passage of apredetermined period of time. j

4. In combination with an audio 4frequency transmission linehaving-input and 'output terminals, an attenuation network comprising anelectron discharge tube having lat leastl an anode, a cathode and acontrol grid, an unbypassed resistor arranged in the space current pathof said tube between the cathode and ground, a condenser connectedbetween the control grid and cathode,.said condenser and resistor beingin series relation, a second resistor and second condenser connected inshunt relation between the control grid and the grounded end of saidresistive impedance, means adapted to apply a negative Avoltage of apredetermined magnitude across. said. last named condenser, said shuntcondenser and resistor providing a time constantnetwork adapted todissipate said charge after the passage of a predetermined period oftime.

5. In a systemof the type employing a source of audio frequencycurrents. an audio current utiquency currents, anaudio frequencyutilization i circuit, an audio transmission line therebetween, Aanattenuation network for the higher audio frequency components of saidcurrents, said network comprising an electron discharge' tulbe circuithaving its' effective input impedance Iin .shunt across said line, saidcircuit having its elemen s so chosen and arranged'that theeilectivemagmtude of said input irripedancelsv proportional to thetransconductance of the electron discharge tube. said input impedancenormally,providing` lization circuit and an laudio transmission linetherebetween; the improvement comprising an attenuation network -for thehigher audio frequency'components of said currents, said networkcomprising an electron discharge tube having its effective input'impedance in shunt across said line, the effective magnitude of saidinput impedance being proportional to the transconductance of theelectron dischargegtube, said input impedance normally providing amaximum attenuation of thel higher audio frequency commonents of saidcurrents, and means for providing a transconductance-reducing bias forsaid tube which is a function of the passage of a predetermined periodof time.

6. In a phonograph systemof the type employing -aI record disc, atransducer device mechanical` ly associated with said disc" and an audiotransmission line connected tsaid transducer; the improvement comprisingmeans for automatically controlling the transmission of higher audiofrequency components over said line, said means comprising an electrondischarge tube having its the transmission of higher audio frequencycomponents through. said line, said means comprising-an 'electrondischarge tube having its linput impedance in shunt across said lineAthereby to provide normal attenuation of the higher audio frequencycomponents of audio currents flowing through said line, a. time constantnetwork connected between the input electrodes of said tube. a source ofnegative potential connected to said time constant network, and meansfor controlling input 'impedance connected to said line to providenormal attenuation of the higher audio frequency components of audiocurrents owing over said line, a time constant network connected.between the input electrodes of' said tube, and means for controllingvthe voltage charging of said time. constant network.

.7. In combination with-a phonograph record K electrical pickup, anaudio frequency current utilization circuit,` and an audio frequencytransmission line therebetween; an attenuation network for the higheraudio frequency components of 'Jsaid currents, said network comprisingan elec-.-

tron discharge tube circuit having its effective input impedanceconnected to said line, said circuit having its constants so chosen thatthe effective magnitude of said input impedance is proportional to thetransconductance of the electron discharge tube, said input impedancenormally providing a maximum attenlation of the higher I audio frequencycomponents of said currents,

and means for controlling the transconductance of said tub@ as afunction of the passage of a predetermined period of time.

8. In combination with a transducer device mechanically associatedwith'a record, an audio transmission line conneted to said transducer,

means for automatically controlling the trans- 'mission of higher audiofrequency components over said line, said means comprising an electrondischarge 'tube having lits input impedance -in shunt across said linethereby to provide normal attenuation of thehigher audio frequencycomponents onfaudio currents vflowing over said line,-

a time constant network connected between the input electrodes of saidtube, a source 'of negative potential connected to said time constantnetl-work, and means for controlling the voltage charging of said timeconstant network by said negative voltage source. Y l i 9. Incombination, a source of audiofrequency currents, an audio frequencyutilization circuit,

an audiorirequency transmission line therebetween, and an improvedattenuation network for the higher audio frequency components of saidcurrents comprising an electron discharge tube circuit havingitseiective input 'impedance in shunt across said line, saidjcircuithaving its elementsso chosen and arranged thatthe eil'ective ,magnitudeof said input limpedance is proportion-a1 to the transconductance of theelectron discharge tube,.said input impedance being condensive andnormally providing attenuation of the higher audio-frequency componentsof said' currents, a 'switch-controlled current source providing atransconductance-'reduclng bias for said tube, and an RC timelconstantcircuit connected Jto vsaid source 'and said tbefor causing said biastobe a function of the passage of n predetermined period of time.

10. In combination, a record disc, a transducer pence-mechanicallyassociated with said disc, an audio transmission line connected to saidtransducer, and improved means for automatically .controlling thetransmission oi' higher audio frequency componentsthrough said linewhich comprises an electron discharge tube having an input capacitancein shunt across said line therehy to provide attenuation of thehigheraudio frequency components of audio currents flowing through saidline, an RC time constant network tronic control over said attenuationwhich'has a decreasing eiect with the passage of time, and progressivelydecreasing the attenuation 'of said Vhigher` audio components inresponse to said decreasing effect of said electronic control as afunction of the time 'during which complete trans- -lation takesplace. 1

12. In a. system of theftypeemploying a source utilization circuit andan audio transmission line therebetween; the improvement comprisinganattenuation network for predetermined audio frequency components ofsaid 4currents, said net .work comprising a reactive impedance in shuntacross said line, a tube for controlling the eilect of said reactiveimpedance, the effective magnitude of said impedance being proportionalto the transconductance of the tube, and means for providing atransconductance-reducing bias for said tube which is a function of thepassage of a predetermined period of time.

Y 13. In a system of the type employing a record I disc, amotor-actuated tuntable supporting said disc, a transducer devicemechanically associated 40` with said ldisc; andi an audio transmissionline connected to saidv transducer; the improvementV comprising meansfor automatically controlling the transmission of higher audio frequencycomponentsover said line, said means .comprising anY electron dischargetube having its input impedance connected to" said line to providenormal attenuation of-the higher audio frequency com,- ponents of audiocurrents owing over said line, `a time constant network connectedbetween the input electrodes of said tube, and means responsive toenergization of said turntable motor for controlling the voltagecharging of said time com' stant network.

` JARRE'I'I L. H ATHAWAY.

`of audio frequency currents, an audio current l

